tests: move to subdir, move to meson

De-clutter the source directories a bit. Ensure tests build with meson, and
remove from autotools in a few places (no need to do things twice).

lib/tests/testdir2/Foo/cur/fraiche.eml

@@ -0,0 +1,10 @@
+From: Sender <test@example.com>
+To: Recip <recip@example.com>
+Subject: search accents
+Date: 2012-12-08 00:48
+MIME-Version: 1.0
+Content-Type: text/plain; charset=utf-8
+Content-Transfer-Encoding: 8bit
+
+line 1: Глокая куздра штеко будланула бокра и курдячит бокрёнка
+line 2: crème fraîche